Regulation of muscle contraction by the thick filaments will be studied at the molecular level. Two control system are known in muscle. In thin filament regulation availability of sites on actin is altered by troponin-tropomyosin, in thick filament regulation sies on myosin are modified by the regulatory light chains. Both systems operate by preventing interactions between actin and myosin in the absence of calcium. Control is exerted by the thin in vertebrate straited muscles, by the thick filaments in molluscan muscles; both regulatory mechanisms operate in many invertebrate muscles. Scallop myosin is particularly useful for the study of subunit regulation since the rgulatory light chains can be fully dissociated from the heavy chains without denaturing the molecule. The regulatory light chains are not needed for contraction, in their absence, however, myosin is no longer regulated. The light chaings can be quantitatively recombined with scallop myosin, and regulation is totally regained. We plan to define the role of the other myosin subunits, the essential light chains in regulation. A regulatory role of the essential light chains is indicated since antibodies specific against this subunit selectively interfere with regulation. We will attempt to remove the essential light chains from myosin with the aid of chaeotropic agents, or exchange these with light chains of non-regulated myosins. We will try to determine interactions and overlap between regulatory and essential light chains by using photosensitive cross-linkers attached to regulatory light chains at known positions. We plan to estimate distance between the two light chain types, and between these light chains and havey chains in the presence and the absence of calcium, ATO and actin by the specific labeling with suitable donor-acceptor pairs. Heavy metal labeled regulatory light cahins will be used as markers to analyse the myosin molecules and the myosin fialments in fiber bundles. We plan to extend the selective labeling techniques of light chain and heavy chains to myofibrils and fiber bundles suitable for X-ray diffraction and tension studies in attempting to develop a preparation that may help to relate chemical and mechanical events.